TCP is connection-oriented. In practice, it is usually necessary to check whether the connection is available. If the connection is unavailable, it can be divided: A. The connection peer is normally closed. B. the connection peer is abnormally closed, including power loss, program crash, and network interruption. in this case, the peer end cannot be notified, so the connection will always exist, wasting resources of the country. The TCP protocol stack has a keepalive attribute, which can actively detect whether the socket is available, but the default value of this attribute is large. The global settings can be changed to/etc/sysctl. conf, plus:
net.ipv4.tcp_keepalive_intvl = 20net.ipv4.tcp_keepalive_probes = 3net.ipv4.tcp_keepalive_time = 60 |
Set in the program as follows:
# Include int keepalive = 1; // enable the keepalive attribute int keepidle = 60; // if the connection has no data exchange within 60 seconds, int keepinterval = 5; // The packet interval when the test is performed is 5 seconds int keepcount = 3; // The number of test attempts. if the response is received for 1st times, the next two times will not be sent again. setsockopt (RS, sol_socket, so_keepalive, (void *) & keepalive, sizeof (keepalive); setsockopt (RS, sol_tcp, tcp_keepidle, (void *) & keepidle, sizeof (keepidle); setsockopt (RS, sol_tcp, tcp_keepintvl, (void *) & keepinterval, sizeof (keepinterval); setsockopt (RS, sol_tcp, tcp_keepcnt, (void *) & keepcount, sizeof (keepcount )); |
In the program, when TCP detects that the Peer socket is no longer available (the detection package cannot be issued, or the detection package does not receive the ACK response package), select returns the socket readable, in addition,-1 is returned for Recv and errno is set to etimedout. |
